1. Field of the Invention
The present invention relates to a hydrostatic transmission (hereinafter, referred to as “HST”). Especially, the present invention relates to a lubricating fluid support system for a lubricated object, typically a bearing, e.g., a roller bearing provided in the HST.
2. Related Art
A conventional HST as disclosed by JP 2012-7675 A includes an input shaft and a plunger block fixed on the input shaft. The plunger block is formed therein with pump cylinder bores, motor cylinder bores and a pair of main fluid passages fluidly connecting the pump cylinder bores to the motor cylinder bores. Pump plungers are reciprocally fitted into the respective pump cylinder bores in the plunger block and a pump swash plate is disposed on one axial side of the plunger block so as to abut against heads of the pump plungers projecting outward from the plunger block, so that the pump plungers and the pump swash plate constitute a hydraulic pump of the HST. Motor plungers are reciprocally fitted into the respective motor cylinder bores in the plunger block and a motor swash plate is disposed on another axial side of the plunger block opposite the pump swash plate so as to abut against heads of the motor plungers projecting outward from the plunger block opposite the heads of the pump plungers, so that the motor plungers and the motor swash plate constitute a hydraulic motor of the HST. The motor swash plate is fitted on the input shaft rotatably relative to the input shaft and is integrated with an output rotary member of the HST, which is drivingly connected to a gear transmission for driving an axle of a vehicle, for example.
In this regard, a needle roller bearing is interposed between an outer peripheral surface of the input shaft and an inner peripheral surface of the motor swash plate so as to support the motor swash plate rotatably relative to the input shaft. More specifically, the needle roller bearing includes needle rollers and a roller retaining ring that holds the needle rollers in the inside thereof. The needle rollers abut against the outer peripheral surface of the input shaft, and an outer peripheral surface of the roller retaining ring abuts against the inner peripheral surface of the motor swash plate. The needle roller bearing has an inside gap space between the roller retaining ring and the outer peripheral surface of the input shaft so as to accommodate the rollers in the inside gap space.
The plunger block is further formed therein with pump valve bores fluidly connected to the respective pump cylinder bores and with motor valve bores fluidly connected to the respective motor cylinder bores. Pump spool valves are reciprocally fitted in the respective pump valve holes and motor spool valves are reciprocally fitted in the respective motor valve holes so that each spool valve is operable to fluidly connect or disconnect the corresponding cylinder bore to and from the main fluid passages. A pump spool cam is fitted to the input shaft between the plunger block and the pump swash plate to engage with the pump spool valves corresponding to the respective pump cylinder bores. A motor spool cam is fitted to the input shaft between the plunger block and the motor swash plate to engage with the motor spool valves corresponding to the respective motor cylinder bores.
The needle roller bearing is provided with an outside gap space around the input shaft between the motor spool cam and the needle roller bearing, and a lubricating fluid spill port is open at the outer peripheral surface of the input shaft so as to face the outside gap space of the needle roller bearing. The lubricating fluid spill port is fluidly connected to a lubricating fluid passage formed in the input shaft. A charge fluid passage for supplementing hydraulic fluid to the main fluid passages is also formed in the input shaft and is also fluidly connected to the lubricating fluid passage in the input shaft. Therefore, surplus fluid from the charge fluid passage flows to the outside gap space via the lubricating fluid passage and port so as to lubricate the needle roller bearing.
The torque and rotary speed of the motor swash plate serve as the output torque and rotary speed of the HST, and they sometimes change quickly. In this case, the lubricating fluid cannot reach the needle roller bearing to make slicks on the needle rollers of the needle roller bearing because of a distance between the inside gap space of the needle roller bearing and the lubricating fluid spill port facing the outside gap space of the needle roller bearing. Further, if the load on the motor swash plate integrated with the output rotary member of the HST is heavy, the rotary speed of an engine driving the input shaft of the HST is reduced so as to reduce the hydraulic pressure in the charge fluid passage in the input shaft, so that the reduced hydraulic pressure is insufficient to fill the lubricating fluid to the outside and inside gap spaces of the needle roller bearing. Such a lack of lubricating fluid supplied to the needle roller bearing wears or damages the needle rollers of the needle roller bearing, thereby shortening the life of the needle roller bearing.